Estimating the glutamate transporter surface density in distinct sub-cellular compartments of mouse hippocampal astrocytes

Abstract

Glutamate transporters preserve the spatial specificity of synaptic transmission by limiting glutamate diffusion away from the synaptic cleft, and prevent excitotoxicity by keeping the extracellular concentration of glutamate at low nanomolar levels. Glutamate transporters are abundantly expressed in astrocytes, and previous estimates have been obtained about their surface expression in astrocytes of the rat hippocampus and cerebellum. Analogous estimates for the mouse hippocampus are currently not available. In this work, we derive the surface density of astrocytic glutamate transporters in mice of different ages via quantitative dot blot. We find that the surface density of glial glutamate transporters is similar in 7-8 week old mice and rats. In mice, the levels of glutamate transporters increase until about 6 months of age and then begin to decline slowly. Our data, obtained from a combination of experimental and modeling approaches, show that there are stark differences in the density of expression of glutamate transporters across different sub-cellular compartments, indicating that the extent to which astrocytes limit extrasynaptic glutamate diffusion depends not only on their level of synaptic coverage, but also on the identity of the astrocyte compartment in contact with the synapse. Together, these findings provide information on how heterogeneity in the spatial distribution of glutamate transporters in the plasma membrane of hippocampal astrocytes my alter glutamate receptor activation out of the synaptic cleft. Author summaryIn this work, we use experimental and modeling approaches to estimate the surface density of the glial glutamate transporters GLAST and GLT-1 in mouse hippocampal astrocytes. The results show that the surface density of glutamate transporters in hippocampal astrocytes is similar in 7-8 week old mice and rats. In mice, GLAST and GLT-1 reach their peak expression at ~ 6 months of age. Their expression, however, varies widely across different sub-cellular compartments. These findings indicate that the identity of the sub-cellular compartment of an astrocyte in contact with a glutamatergic synapses, in addition to the extent of its synaptic coverage, is a main factor to limit glutamate spillover and receptor activation at a distance from the release site.